Solutions involving Multiple Input Multiple Output (MIMO) antenna configurations, in which both the transmitter and the receivers have multiple antennas, are being considered for use in wireless communications networks to improve system performance in terms of peak data rate, coverage and capacity.
In the general case, in such a system, both the transmitter and the receivers have multiple antennas. This results in a number of possible radio channels, between each transmitter and receiver antenna. A channel matrix H can be defined to characterize all the channels. If N transmit antennas and M receive antennas are used the size of the channel matrix H will be M×N. H generally varies over time.
In the case when the channel is known to the receiver but not to the transmitter, data is transmitted uniformly in all directions, and the channel capacity can be expressed as
                    C        =                              lg            2                    ⁡                      (                          det              ⁡                              (                                  I                  +                                                            ρ                      N                                        ⁢                                          H                      ·                                              H                        *                                                                                            )                                      )                                              (        1        )            where N is the number of antennas at the transmitter, ρ is the total received transmit power divided by the noise power at the receive side, I is the identity matrix, and * is the Hermitian operator.
It is well known that under fading conditions with statistically uncorrelated propagation channel, the channel capacity measured in bits per channel use scales (from an information theory point of view with fixed average transmit power) on average asCMIMO=CSISO·min{M, N},  (2)where CSISO is the channel capacity for (traditional) single transmit single receive antenna communication (Single Input Single Output), i.e.CSISO=1g2(1+SNRSISO),  (3)
where SNRSISO is the SISO signal to noise ratio and CMIMO is the resulting MIMO channel capacity. When M=N the channel capacity is N times the SISO channel capacity, i.e.CMIMO=N·CSISI  (4)
Note that SISO communication has a logarithmic relation for channel capacity with respect to SNR (see eq.(3)). The benefit from MIMO transmission with multiple data streams is that instead of using all power in one stream, multiple parallel streams are used with slightly lower SNR instead. In this way a capacity multiplication is obtained instead of a logarithmic increase in capacity.
So far, the discussion has been concerned with the case that the transmitter does not know anything about the channel matrix H. For the case when the transmitter has knowledge about the channel, performance can be further improved by transmitting data streams with different powers on the different modes of the channel. In addition, the channel knowledge can also be used to reduce the terminal complexity when demodulating and decoding the received signals.
An extensive overview of MIMO is found in A. Goldsmith, S. A. Jafar, N. Jindal, S. Vishwanath, “Capacity Limits of MIMO Channels”, IEEE Journal on Selected Areas of Comm., VOL. 21, NO. 5, JUNE 2003.
One recent, alternative, way of handling communication in MIMO systems is opportunistic MIMO, which is also sometimes called multiuser diversity MIMO. The idea is that one may, for each of potentially many channels, send not all MIMO streams (hereafter called MIMO subchannels) to a single user, but instead distribute the MIMO subchannels over several users. This can be accomplished in an opportunistic manner by selecting users based on Carrier to Interference Ratio (CIR) information fed back from the receiving users. In MIMO, CIR information is fed back for each MIMO subchannel. The more receivers present, the more likely it will be that one finds “good” channels, and this is guaranteed in a statistical sense. The opportunistic MIMO architecture is illustrated, for example, in W. Rhee, W. Yu and J.M. Cioffi: “Utilizing Multiuser Diversity for Multiple Antenna System,” Proceedings of IEEE Wireless Communication and Networking Conference (WCNC), p 420-425, September 2000, Chicago, USA.
Opportunistic MIMO makes use of the fact that with a large number of users, it is likely that the MIMO channels may have realisations in which one or several MIMO streams may be received with high quality by one or more users despite the fact that no CSI is used to predict the signals at the transmitter. This is achieved even when using simple non-optimal demodulation methods such as zero-forcing. In the end, it is the base station that determines which MIMO subchannel to use for which user. In addition to zero-forcing, other well-known demodulation methods such as MMSE, Successive Interference Cancellation (SIC), Parallel Interference Cancellation (PIC), or other Multi user detection schemes (MUD) can be used.
Opportunistic MIMO is feasible in situations where a large number of users are involved and data is pending transmission to them. Performance in situations where only a few users are involved is lower.